Vertical insulated gate bipolar transistors (IGBTs) which are composed of a plurality of transistor cells connected in parallel are known in the field of power semiconductor components. In such vertical IGBTs the electron current flows through the semiconductor die in a vertical direction from a top surface (or front side surface) to a bottom surface (or back side surface) of the semiconductor die, whereas hole current flows in the opposite direction. In case of an n-channel IGBT, a p-doped collector region (also referred to as p-emitter region in case of n-channel IGBTs) is located at the back side of the semiconductor die, where it is electrically connected to the collector electrode. A p-doped body region and an n-doped source region are disposed at the front side of the semiconductor die in each transistor cell, whereby the both regions of each individual transistor cells are connected to the emitter electrode. It is further known to provide a so-called field stop region, which adjoins the p-doped collector region at the back side of the semiconductor die. The field stop region is doped with dopants of the opposite doping type than the collector region. Furthermore, the concentrations of dopants in the field stop zone is significantly higher than in the so-called drift region which extends through the semiconductor die from the field stop region at the back side of the die to the body region at the front side of the die.
Generally field stop regions may be provided, for example, in semiconductor components which have, in an off-state, a vertical pnp-structure with a blocking pn-junction (between body region and drift region) at the front side of the semiconductor die. When the applied blocking voltage is so high that the electric field (or the space charge region, also referred to as depletion region) would extend down to the back side of the semiconductor die, then the mentioned field stop region is needed to avoid a “punch-through” of the electric field which would result in a breakdown of the transistor. The field stop region is somewhat more heavily doped than the neighboring drift region, and reduces the electric field at the back side of the semiconductor die and avoids the mentioned punch-through.
A peak of the electric field can occur in the field stop region when the transistor is in short-circuit operation, i.e. when the transistor operates at a high collector-emitter voltage while a gate-emitter voltage greater than the threshold voltage is applied to the transistor's gate electrode. Such a peak of the electric field can lead to a local breakdown of the semiconductor component and thus to a destruction of the IGBT. Furthermore, a too low electric field at the front side of the drift region (adjoining the MOS channel and the body region) can stimulate the formation of current filaments when the transistor is in short-circuit mode. Such current filaments can lead to local hot-spots and thermal destruction of the IGBT.